Deionized water simply means that certain dissolved solids have been removed from it. The result is water with very low ionic content. An ion is simply another name for a charged atom such as Na+ (sodium) or Cl- (chloride). These minerals are present in municipal feed water supplies. They are harmless to humans unless they grossly exceed the maximum allowable limits allowed by the EPA.
What is important to know is that there are different ways to deionized water and different methods to achieve it. Any water that has ions removed is considered deionized.
There are three ways to deionize water:
An under the sink Reverse Osmosis system will remove a large quantity of ions through a membrane separation process, therefore the water it produces is considered deionized. It is safe to drink water that has been treated by an RO system as long as the feed water comes from a safe municipal source. Usually an under the sink RO unit comes with a sediment filter and a carbon filter, which helps to protect the RO unit and also improves the taste of the water.
Distillation produces distilled water by boiling the water and then condensing the steam into liquid form. The water is then collected in a clean container.
Water that passes through an ion exchange column will have incoming ions exchanged for hydrogen and/or hydroxide ultimately producing (nearly) pure H20. The performance of ion exchange resins is determined by measuring the electrical conductivity of the water. The lower the conductivity, or higher the resistivity, the more ‘demineralized’ the water is. This is because charged atoms (ions) conduct electricity. Ion exchange resins will eventually become exhausted and produce water with a higher conductivity value (and more ions present in the water). The lowest possible conductivity for water to reach is 0.056 µS (micro-siemens) or 18.2 Meg-ohm resistivity. Therefore, deionized water with very low conductivity will have extremely low levels of ions or minerals present.
The World Health Organization investigated the health effects of demineralized water in 1980, and its experiments in humans found that demineralized water increased diuresis and the elimination of electrolytes, with decreased serum potassium concentration. Magnesium, calcium and other nutrients in water can help to protect against nutritional deficiency. Recommendations for magnesium have been put at a minimum of 10 mg/L with 20–30 mg/L optimum; for calcium a 20 mg/L minimum and a 40–80 mg/L optimum, and a total water hardness (adding magnesium and calcium) of 2–4 mmol/L. At water hardness above 5 mmol/L, higher incidence of gallstones, kidney stones, urinary stones, arthrosis, and arthropathies have been observed. For fluoride the concentration recommended for dental health is 0.5–1.0 mg/L, with a maximum guideline value of 1.5 mg/L to avoid dental fluorosis.
Likewise, an ion exchange column is intended for industrial use. The ion exchange tank will remove chlorine as well which leaves a large portion of the column without an oxidant to destroy bacteria and other harmful pathogens. Therefore, you may achieve water with a low mineral content, but a large bacteria count.
Some argue that a complete void of minerals and ions such as calcium that play key roles in biological functions such as in the nervous system and should not be removed from drinking water. The Journal of General Internal Medicine published a study on the mineral contents of different waters available in the US. The study found that "drinking water sources available to North Americans may contain high levels of calcium, magnesium, and sodium and may provide clinically important portions of the recommended dietary intake of these minerals". It encouraged people to "check the mineral content of their drinking water, whether tap or bottled, and choose water most appropriate for their needs". Likewise, the consumption of water with normal mineral levels is associated with beneficial cardiovascular effects. As noted in the American Journal of Epidemiology, consumption of ‘hard’ drinking water (water that is not deionized or demineralized) is negatively correlated with atherosclerotic heart disease.
The question of whether or not to drink deionized (demineralized) water is up to you. Water produced by reverse osmosis is considered safe to drink. Distilled water is still in debate. Drinking water from a deionized water column (such as Puretec Industrial Water provides) is not recommended as it is not intended for this purpose.